1. Field of the Invention
The present invention relates generally to a power converter, and more specifically, to a power converter having a switch controller controlling the switching cycles at start-up and/or power on reset (POR).
2. Description of the Related Art
Power converters typically require error circuitry that provides an “error” signal between the output voltage of the power converter and a reference voltage, in order to regulate the output voltage. The error circuitry should provide a magnitude and a sign (positive or negative) of the output voltage relative to a reference voltage. The power converter can use the error signal to properly regulate the output voltage by increasing or decreasing the amount of power delivered to the output of the power converter in response to the error signal.
Conventional power converters typically generate an error signal by sensing the output voltage as an analog value, deriving the difference between the sensed output voltage and the reference voltage as an analog value and amplifying it. Conventional power converters may also use an analog-to-digital converter (A/D converter) to generate the error signal depending upon the control scheme. Other conventional power converters may use analog error amplifiers to generate the error signal.
In many conventional isolated switching power converters, the output voltage is directly sensed on the secondary side of a transformer and compared to a reference voltage fixed so that the output voltage of the power converter is regulated to a target level. Alternatively, other conventional isolated switching power converters do not sense the output voltage directly, and instead sense a voltage on the primary side of the transformer. The sensed primary-side voltage is compared to a reference voltage fixed so that the output voltage of the power converter is regulated to a target level. These isolated switching power converters are commonly referred to as primary-only feedback converters.
In the case of primary-only feedback converters, there may be situations where during certain switching cycles, the output voltage may differ dramatically from the regulated target voltage as set by the reference voltage. One example of this occurrence is during the initial start-up of the switching power converter. The initial start-up phase is especially difficult for primary-only feedback converters because they do not directly sense the output voltage. When the output voltage differs dramatically from the regulated target voltage during start-up, the sensed primary side voltage provides an inaccurate representation of the output voltage. As a result, conventional primary-only feedback converters may be slow to reach the regulated target voltage after start-up. The problem may worsen when the secondary output stage of a primary-only switching converter is coupled to an output filter stage that includes high capacitance electrolytic capacitors, thus further slowing the startup cycle. If the controller of the primary-only switching power converter fails to raise the output voltage in a controlled manner from 0 VDC to the regulated output voltage level within a maximum amount of time, damage may be caused to any electronic device(s) connected to the power converter.
Another major function of the controller is to sense and detect abnormal and/or fault conditions, and to provide a safe operating mode for the switching converter until the fault condition is removed. At the start-up mode, the output voltage, typically starting at 0 VDC, is significantly lower than the desired regulated output voltage level. The capacitive load based on the output filter capacitors also impacts the rise time of the output voltage. The condition of the output voltage being significantly lower than the desired regulated output voltage level can also be observed in a fault condition where there is a short circuit of the output of the switching converter. As a result, conventional primary-only switching converters are susceptible to false fault detection. This may prevent the switching converter from start-up even when a fault condition does not exist.
One conventional solution to the above described problems is to configure the controller to deliver the maximum amount of energy to the secondary until the output reaching the regulation point is detected. However, this conventional solution is still problematic because it may cause output voltage over-shoot, and subsequent output voltage ringing.